Piezoelectric transducers



Feb. 25, 1958 H. l. OsHRY ET AL 2,824,980

PIEzoELEcTRIc TRANsDUcERs Filed March 14, 1952 'if' i F/Cr. /f7

PIEZOELECTRIC TRANSDUCERS Howard I. Oshry and John W. Schell, Erie, Pa., assignors to Erie Resistor Corporation, Erie, Pa., a corporation of Pennsylvania Application March 14, 1952, Serial No. 276,501 1 Claim. (Cl. S10-8.3)

This invention is intended to increase the output or the uniformity of output of piezoelectric devices subject to impact.

Where the device is a brittle material, such as barium titanate ceramic, localized impact may fracture the ceramic. lt has been found that sandwiching the ceramic between metal plates increases not only the total output, but also the uniformity of the output. Because of the diii'iculty of soldering the metal plates to the metalized electrodes with which the ceramic is customarily provided, a conductive cement is utilized to attach the metal plates.

In the drawing, Fig. l is a plan view of a barium titanate piezoelectric transducer; Fig. 2 is an edge view; Fig. 3 is a circuit diagram; Fig. 4 is an enlarged fragmentary section through the transducer; Fig. 5 is an enlarged side view of a phonograph pick-tip; and Fig. 6 is a section on line 6 6 of Fig. 5.

The invention is shown applied to a barium titanate piezoelectric transducer consisting of a barium titanate ceramic disk 1 having metalized electrodes 2 and 3 on opposite faces thereof. The metalized coatings are applied by the technique for decorating ceramics and in general consist of sintered silver pigment particles fused to the ceramic by a flux. The electrodes 2 and 3 terminate short of the periphery of the disk providing an insulating band 4 which prevents ashover. After the electrodes 2 and 3 are applied to the fired ceramic disk, a polarizing voltage is applied between the electrodes activating the ceramic so that it exhibits piezoelectric properties as described in Patent No. 2,486,560, Gray. As explained in the patent, a mechanical stress on the barium titanate causes an electrical voltage to appear between the electrodes 2 and 3.

In Fig. 3 is shown an application of the barium titanate transducer in which the transducer is subject to impact to create a voltage which actuates a signal device such as a lamp 5. In this application, the barium titanate transducer is rested on a stable support 6 with the electrodes 2 and 3 connected across the lamp 5. An impact from a tool 7 generates a voltage across the electrodes 2 and 3 sufficient to light the lamp. Obviously,

the lamp is merely illustrative of other devices which might be actuated by generated voltage. In this application, it has been found that the impact sometimes causes the ceramic disk to fracture, which does not produce the electrical voltage across the electrodes 2 and 3 necessary to light the lamp 5. It has been found that the voltage output of the transducer will be larger and more uniform if metal plates 2a and 3a are attached to the correspondingly numbered electrodes. The metal plates have the function of distributing the impact energy over the entire electroded area of the disk, thereby preventing the localized strains which might result in fracture. The metal plates 2a and 3a may be attached to the electrodes by soldering, but because the silver electrodes 2 and 3 are difficult to solder to without breaking the bond between the electrodes and the ceramic disk,

nited States Patent O 2,824,980 Patented Feb. 25, 1958 ICC it is preferable that the metal plates 2a and 3a be attached to the electrodes by a conductive cement which may, for example, consist of powdered graphite and an epoxy resin solventless varnish having the property of setting up in situ by addendum polymerization. The temperature required to polymerize or set up the solventless varnishes can be substantially lower than the Curie point of the barium titanate ceramic so that the polarization of the ceramic is not destroyed by the heat necessary to set up the solventless varnish cement. This means that when the plates 2a and 3a are attached to the electrodes by the conductive cement, the barium titanate ceramic disk may be first activated or polarized and thereafter the plates 2a and 3a attached by the conductive cement. The heat of curing is low enough so that the polarization is not destroyed. The curing temperature of the cement can be as high as C. without destroying the polarization. In contrast to the conductive cement, when the plates 2a and 3a are soldered to the silver electrodes 2 and 3, the soldering temperatures are well above the Curie point and the ceramic disk cannot be polarized until after the plates 2a and 3a are attached. Then, because of the possibility that the soldering operation may have destroyed the bond between the silver electrodes 2 and 3 and the ceramic, it is entirely possible that the subsequent polarizing operation may not produce the desired output characteristics.

Because piezoelectric transducers ordinarily feed into load resistances of substantial size, for example, of the order of l0,000-l00,000 ohms, it is not necessary that the conductive cement indicated by the layer 8 in the enlarged section, Fig. 4, have as low conductivity as usually found in metals. A specific resistance of l() ohms per centimeter cube is a perfectly satisfactory value for the conductive cement 8. This layer of conductive cement is of relatively large area and is very thin so that the resistance between the metal plates 2a or 3a and the correspondingly numbered electrodes can easily be of the order of from 1 to 2 ohms, which is negligible compared to the load resistance ted by the piezoelectric device. The conductive cement further tends to till in some of the surface irregularities of the ceramic so that higher mechanical loads can be applied to the plates 2a, 3a when held in place by the conductive cement.

ln Figs. 5 and 6 are diagrammatically illustrated the application of the conductive cement to a phonograph pick-up. The phonograph pick-up has one end suitably mounted or fixed in a support 9 forming part of the structure of the tone arm and has the other end supplied with a metal cap i0 having a socket 11 receiving a phonograph needle 12 which is suitably clamped tnerein by a set screw 13. The structure so far described is diagrammatic and is only for the purpose of illustrating the principle. lhe piezoelectric element of the pick-up comprises two elongated barium titanate ceramic plates or bars 14, 15 which have on opposite faces thereof metalized electrodes 16, 17, and 18, 19. The dimensions of the ceramic plates 14, 15 are greatly exaggerated, as these ordinarily will have a thickness of approximately 10 to 12 mils. Each of the ceramic plates or bars 14 and 15 is polarized by a voltage applied between its electrodes. In the case of the plate 14, the polarizing voltage is applied between the electrodes 16 and 17, while in the case of the plate 15, the polarizing voltage is applied between the electrodes 18 and 19. After the polarization, the plates 14 and 15 are arranged backto-back; that is, so that the direction of polarization is in each case from the center electrodes 17, 18 outward toward the outer electrodes 16, 19. The purpose of this arrangement of the plates 14 and 15 is to make the piezoelectric voltages generated by the lateral movement of the phonograph needle 12 act in the same direction or polarity from the center electrodes to either of the outer electrodes so that the outer electrodes 16 and 19 can be connected together to feed an output lead 21 and the other output lead 20 can be taken oti from the center electrodes 17, 18. This back-toback arrangement is common in phonograph pick-ups. It requires an electrical and a mechanical connection of the adjacent electrodes 17, 18, which has been heretofore effected by soldering before the polarization of the ceramic plates 14, 15. As explained above, the soldering operation tends to destroy the bond between the electrodes and the adjacent ceramic and if the bond to either ceramic plate is destroyed, the assembly will not be satisfactory for phonograph pickups,

In accordance with the present invention, the diiculty of connecting the adjacent electrodes 17, 18 is overcome by the use of an intermediate layer 21 of the epoxy resin solventless varnish-powdered carbon conductive cement. This conductive cement can be used to connect previously polarized elements so that only the plates which are satisfactory will be assembled into the completed pick-up. Where the adjacent electrodes 17, 18 have been connected by soldering, the polarizing could not take place until after the soldering operation, which necessarily resulted in a higher scrap loss than with applicants construction where the polarized elements can first be tested to eliminate the defective elements and then cemented together by the conductive cement which not only makes the necessary electrical connection but also the required mechanical connection.

The conductive cement bond due to its flexibility cuts down the change in characteristics which ordinarily accompanies mechanical or thermal cycling.

What is claimed as new is:

A transducer subject to impact comprising a piezoelectric body of barium titanate having opposite faces coated with metallized electrode coatings, a metal plate overlying each electrode coating for distributing the irnpact energy over the entire electroded area of the body, said metal plates being both electrically and mechanically connected to the underlying electrode coating by an intervening conductive cement film of a mixture of carbon and epoxy resin solventless Varnish polymerized by addendum polymerization.

References Cited in the tile of this patent UNITED STATES PATENTS Re. 20,680 Sawyer Mar. 29, 1938 1,495,429 Nicolson May 27, 1924 2,374,637 Hayes Apr. 24, 1945 2,392,429 Sykes Jan. 8, 1946 2,456,995 Robinson Dec. 21, 1948 2,479,926 Gravley Aug. 23, 1949 2,484,950 Jaffe Oct. 18, 1949 2,486,150 Gravley Oct. 25, 1949 2,497,108 Williams Feb. 14, 1950 2,498,210 Kilgour Feb. 2l, 1950 2,528,932 Wiles Nov. 7, 1950 2,569,920 Buck Oct. 2, 1951 2,589,403 Kurie Mar. 18, 1952 2,640,165 Howatt May 26, 1953 2,640,889 Cherry June 2, 1953 OTHER REFERENCES Properties of Conductive Plastics, in Electronics, October 1949, pp. 96-99. 

